Alumina (Al.sub.2 O.sub.3) has been a favored ceramic substrate material for electronic circuitry where long life and high reliability are required. However, for certain high performance applications, sintered alumina substrates have proven less than satisfactory.
While an alumina body can be ground to a smooth finish, it undergoes high shrinkage (about 18%) on firing. Sintered alumina has a relatively high dielectric constant of about 10, which restricts line spacing and may lead to signal delays and noise in operation. A relatively high coefficient of thermal expansion (about 65.times.10.sup.-7 /.degree.C.), as compared to that of silicon chips (about 35.times.10.sup.-7 /.degree.C.), can make it difficult to seal such chips to an alumina substrate. Finally, high firing temperatures (about 1600.degree. C.) are required for co-sintering. This limits the metals that may be employed to molybdenum and tungsten, and excludes silver, copper and gold.
Accordingly, a search has been made for substrate materials having more compatible properties than sintered alumina. In particular, materials having coefficients of thermal expansion not over about 45.times.10.sup.-7, and hence more closely matched to silicon, and a lower dielectric constant than alumina, have been sought.
Glass-ceramic materials, in particular the cordierite-type, have received considerable attention. Some of the history is reviewed in co-pending application Ser. No. 07/238,574 filed Aug. 31, 1988 and assigned to the assignee of this application. This co-pending application discloses modified cordierite compositions that have superior properties to those of alumina, and that can be sintered below 1,000.degree. C. The latter property permits co-sintering, that is, firing noble metal circuitry and sintering the substrate in one heat treatment.
Aluminum nitride (AlN) and mullite are other ceramic materials that hold considerable promise as substrate material. In addition to a compatible coefficient of thermal expansion, AlN has an exceptionally high thermal conductivity. This permits dissipation of heat from hot spots that tend to develop during circuit operation, and that might otherwise damage the circuitry. U.S. Pat. No. 4,719,187 (Bardhan et al.) describes AlN and an improved method for its production.
The advent of these lower expansion substrate materials has created a need for a new dielectric having compatible expansion characteristics in order to build hybrid circuits involving crossovers.